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Are there non ballistic/pyrotechnic parachutes for airplanes?
Is there an aircraft parachute system that doesn't use brs / pyrotechnic ejection for its deployment?
I'm thinking something simple like ones used in skydiving or with a  spring mechanism as used in this drag chute:

(source)
video
NOTE: The question is limited to parachutes for deployments in the air. (I.e.: Parachutes for braking purposes on the ground are excluded)

I thought there was only one certified whole aircraft parachute system?

@AnonymousPhysicist if you are referring to Cirrus they are not the only one, they made it popular, but you can even put a parachute on a 182 if you want: https://www.youtube.com/watch?v=Eo7ZmcAWxwk

@AnonymousPhysicist Cirrus planes (SR20/22 and SF50) have a parachute uniquely certified and factory installed as part of the type certificate. A number of other planes, including the Cessna 150, 172/182 have a system certified as a third-party aftermarket modification under the supplemental type certificate (STC) process.

@user71659 Right, and are all those type certificates really for the same product?

BRS makes (almost all of) them. The product varies based on gross weight but they are very similar.

Your title said "airplanes", but in your body of text you said "aircraft".

Is there an aircraft parachute system that doesn't use brs /
pyrotechnic ejection for its deployment?

Hang gliders and paragliders are ultralights, which are considered "aircraft" in the US.  (See ASE answer In the United States, are ultralights legally considered aircraft?. )
Hang gliders and paragliders usually have hand-tossed parachute deployment bags, located on the pilot's harness.  These parachutes lower the glider and pilot together as one unit.
Here is a link showing deployment of such a parachute during an incident where a hang glider accidentally got lifted up into a cloud and entered a high-G turn and the pilot was concerned about loss of consciousness, and failure of the pilot harness, as well as breakup of the glider itself.  (The pilot had no idea which direction to roll the glider to exit the turn.)  The video is set to open at 4:28 which is a few seconds before the parachute deployment.  You'll have to play around with pausing the video to hope to glimpse the actual hand-thrown deployment bag, but you can easily see the long parachute bridle.

Of interest is the fact that the vario seems to be occasionally playing some "lift" tones even immediately after the deployment.  Wow.  Talk about "cloud suck".

Also of interest--look at the direction of the parchute bridle relative to the camera at 4:37 or 4:48.  Clearly the glider is inverted here.  The pilot outweighs the glider, thus the chute's pull is not sufficient to pull the pilot to a position above the glider, the pilot is below the glider. The glider is inverted.  Around 5:11 things change, and the pilot ends up above the glider, with the (now upright) glider hanging below him.

It would be sweet if the video continued till landing or at least till exiting the cloud, but unfortunately it does not.


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Which aircraft has the simplest folding wing mechanism?
From all the different folding wing mechanisms, which airplane has one of the simplest / easiest mechanisms?

I think this is my favourite folding wing mechanism: https://www.foldnfly.com/15.html#Spin-Plane

A single sweep with one huge hinge-pin (and leaking fuel tanks) was a characteristic of the F-111 (Aardvark). The B-1 and F-14 shared similar sweep wings. Now "sweep" may be something different than "fold" you are asking, but the sweep was fairly simple in concept.

Sweep is also valid answer. Now there are not many(only come to mind the ones you mentioned) or are there more? It seems folding is easier/simpler design than sweep.

@David C. Rankin This should be an answer. I'd vote for it.

Yes, just a bit confused by the "fold" term. The sweep and fold generally are done for different purposes. The sweep is done to alter the flight-envelope of the aircraft allowing for flight at slower speed for takeoff/landing. The "fold" is generally done for space saving during transport/storage like on a carrier. Arguably both fold, but just depends on what the questioner is looking for. The answer captures the simplest by far.

There are many planes with folding wing structures. Some like the F/A-18 fold a small part of the overall wing and similar is with the 777X
From what I got to know, the aircraft with the simplest folding structure (experimental and kit planes included), would be this Onex Kit plane by Sonex Aircraft
It has a really simple system,

The folding wing feature provides one handle this is pulled to release two spar pins, one in the main spar, one in the rear spar. With the pins released, the wing is simply folded by-hand into the up position. The wing fold handle has a spring loaded locking pin that keeps the wings locked for flight. The wing fold & unfold/lock process takes only about 30 seconds per wing!

This is a quote from their website which shows how simple the system is. It's not even electrical or hydraulic so I'm guessing it has to be the simplest.

This plane in flight

Wings folded with folding mechanism visible

Out of curiosity, is the handle inside the cockpit or on the wing?

Can you imagine if the handle was in the cockpit right next to the throttle :D perhaps there is some kind of mechanism that prevents the pins to be released during flight, like when the engine is on, or something like that.

If I were designing such a thing, I'd probably lean towards something like the wing loading forcing the pin into an immovable position, rather than making it depend on the engine. Make it physically impossible to move the wing while in flight.

F-18 folds almost about a third of the wing, lerx excluded.

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Is a Tandem more efficient than a Biplane?
Considering both airplanes have 2 wings I wonder what are the pros and cons of each one and if a tandem design like the Quickie Q2 is more efficient than a biplane like the Steen SKYBOLT?

(wikipedia)

(wikipedia)

Considering both airplanes have 2 wings

The premise of your question is not correct: the Q2 has one wing and one horizontal empennage while the Skybolt has... well, one wing and one horizontal empennage - they are actually the same
In the Q2 (which is indeed a tandem design) the horizontal empennage has grown in size so much that it has become as big as the wing while in the Skybolt (which is indeed a biplane) the wing has been "divided" in two and put one half over the other.
Even if with two visually very different results, the core idea behind both designs is the same i.e. to split the total lift between two lifting surfaces in order to reduce the induced drag: being the induced drag proportional to $L^2$, if each lifting surface generates now $L/2$ then the relevant induced drag becomes $(L/2)^2=L^2/4$; having two lifting surface, the total induced drag is therefore $2 \times L^2/4=L^2/2$ i.e. half of what we get with only one wing, nice!

Obviously this is not the whole story.
To get each lifting surface half of the total area, their span and/or chord has to become smaller. But shorter wingspan means higher induced drag (which is the opposite of what we wanted to obtain in the first place) while shorter chord means smaller Reynolds number and again higher (viscous) drag.
In addition to that, each of the two designs has other compromises that further reduce the gain in induced drag:

In the tandem design, to maintain stability the CG must be closer to the forward lifting surface than the rearward; therefore the forward surface generates more lift than the rearward; this not 50-50 split of lift reduces the previously calculated ideal gain in the induced drag. Plus we have a rearward surface which is as big (and heavy) as the front one but that generates less aerodynamic forces: it is basically oversized. Furthermore, the downwash of the forward surface impinges on the rearward surface; this is not intrinsically a bad thing but makes the airplane unstable: let's say that, for whatever reason, the lift on the forward surface increases $\rightarrow$ the downwash increases $\rightarrow$ the rearward surface sees a lower AoA and generates less lift $\rightarrow$ we get higher lift forward and lower lift rearward $\rightarrow$ this creates a nose-up moment that further increases lift in the forward surface and reduces lift on the rearward surface $\rightarrow$ the airplane keeps pitching-up in an unstable way. Note that the Q2 partially solves this pitch-up problem setting the two surfaces at different heights.

In the biplane design, the two lifting surfaces are connected via bracing which increases drag. Aerodynamic interference among the upper and lower wing also increases drag.

If the Q2 isn't a tandem wing, then what is?

@CamilleGoudeseune: I was just stretching the definitions a bit

The smaller chord wing will become heavier while struts and bracing make the biplane wing much lighter. Both effects also influence induced drag, because they determine how much lift needs to be generated and must be taken into consideration.

The glide ratio of both types is less than 10, with the "quickie" perhaps slightly better.
This all points towards fuselage and wingtip interference.  The glide ratios are also similar to monowing types such as the Cessna 152.
No free lunch here, long slender wings (high aspect ratio) are needed for better efficiency.
Mounting the top wing in a parasol fashion may help the "quickie", as would streamlining the bi-plane with retractable landing gear.

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Will you be able to hear a supersonic aircraft flying away from you?
I'm wondering if you could be able to hear the sound emitted by an aircraft say traveling at Mach 2 away from you.
I wondered if it might shift the emitted frequencies beyond the audible range. Would you be able to hear it?

XKCD's "What If?" has a nice article about a supersonic Mach 2 sound source: https://what-if.xkcd.com/37/

Ideally, avoid asking a question where the title asks it one way (will you be able to hear it) and the text asks it another (would it shift...beyond the audible range) as answers starting with (say) "Yes" are ambiguous. Your only answer so far starts with "yes," so I'd adjust the text to fit the title. (I've suggested an edit that does that.)

A tangential question I once heard was, when you're flying in a supersonic plane do you hear the plane? The answer was yes, in a way. You'll hear the wind noise, the vibration of the engines and other materials, etc.

Yes, actually you can only hear a supersonic aircraft after it has passed over you and is now flying away from you since it is moving faster than the sound moving towards you. The sound waves will still propagate in all directions and will eventually reach you:

The frequency will be shifted according to the Doppler formula:
$$ f = \frac{c \pm v_r}{c \pm v_s} f_0 $$
Here, $v_r$ is the the speed of the receiver ($v_r = 0$ for you standing still) and $v_s$ is the speed of the sender. It is added when the sender is moving away from you. In your case of $v_s = 2c$ you would get
$$ f = \frac{c}{c + 2 c} f_0 = \frac{1}{3} f_0 $$
The audible range for a typical human is about 20Hz to 20kHz. So you would still be able to hear any $f_0$ greater than about 60Hz after the Doppler shift. The following image shows the frequencies emitted by a Boeing 747 as measured from the ground:

(W.L. Rubin, The Generation and Detection of Sound Emitted by Aircraft Wake Vortices in Ground Effect)
There are a lot of contributions from the aircraft above 60Hz, which should make it clearly audible. Of course an aircraft flying at Mach 2 will be different than a 747, but the general spectrum should not change too much.
Note: the formula above does not work while the aircraft is moving towards you because the resulting frequency would become negative. This is because sound emitted at a later time reaches you before the sound emitted at an earlier time because the aircraft has moved more towards you than the sound:

I think it's awesome that the formular gets negative frequencies, and that acutally means something (sound is inverse)

@Christian Negative frequencies aren't really a thing, but one can interpret it that way :)

@Christian: A better interpretation is that the result/value of the function isn't physically meaningful over that part of its domain, and therefore you need to special-case it.  Sometimes physics does work like that, e.g. optics where a negative image distance can be physically meaningful: a lens or mirror forming a virtual (behind the mirror) vs. real (projected onto a screen) image.

But here notice that the formula has a singularity (f * c / 0) at v_s = -c: the formula breaks down and we need to consider what happens to real non-ideal fluids as sound and shock waves "pile up" around an object moving exactly at the speed of sound.  You don't just get all the sound energy building up forever delivered in one huge burst.  For an object moving faster than sound, it might actually sort of invert the wave-form, if you ignore all the added noise from moving faster than the speed of sound in real air.  (shock waves are loud)

in theory, couldn't the plane fly in circles about your location, neither getting closer nor farther away; and you'd still hear it?


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Can ailerons be used to control yaw and pitch
Is it possible for an aileron to be used to control the roll, pitch and yaw of a vtol drone

You say "VTOL" drone, I assume you want the ailerons to control y/p/r when in forward flight? I don't see the ailerons having any effect at low speeds during VTOL transition.

Should not you confuse the reader with your vtol. You should only say airplane as too many type of drone, from an airplane-like to a helicopter-like.

Depending on the geometry of the vehicle, up to a degree yes. However, with a classic aileron setup and no rudder, it might be impossible to control roll and yaw independently. A split flap (up and down on same wing) would be required to more effectively control yaw, as that can in theory generate drag on one side without much affecting lift. The B2 bomber sports a similar configuration.

I once built a model glider with a little dihedral. With coordinated aileron and rudder input it would turn nicely, but when only the ailerons were deflected, the adverse yaw made it yaw without any tendency to roll. Due to the lack of bank it would continue on the same course, but now with considerable sideslip.

@PeterKämpf That’s interesting! The asymmetric lift due sideslip must have cancelled out with asymmetric lift from aileron deflection. Coincidence or design?

Pure coincidence!

Yes sir, we can see the dihedral stability (to roll) was stronger than than aileron tendency to create roll. My guess it was also a high wing.  So much is learned from these little models.

Yes, and rolling back into low aileron from "dihedral effect" from side slip (as I learned from you).

@PeterKämpf Nice. A little unfortunate for that glider, but great experience to make first hand! Have edited my answer to not categorically rule out yaw control with normal ailerons only (although in your example you still needed the rudder for independent roll and yaw control).

I would straighten it up with rudder to see difference in side slip roll and dihedral stability counterforces.  Let's not forget rolling contribution of tail.  Have noticed the 1 meter carbon composite "Elf" glider has about 2/3 up, 1/3 down.  I hope to fly it next to my own designs.

@RobertDiGiovanni: You guessed correctly, even though the fuselage was rather small - this was not a scale glider. And with coordinated rudder it flew very nicely.


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Can you identify this fighter aircraft?
If someone can identify this aircraft, please tell me which aircraft this is?

... also known as the source of a lot of F-35 design data.

This totally looks like someone photoshopped in pumpkins for those downward facing engines.

Jack-o-lanterns for afterburners.

It's a Russian YAK-141

This was the VTOL prototype meant to compete with the Harrier and perform carrier-based work (note: edited for clarity)

Designers from the Yakovlev bureau found out, that the double engine scheme of the Yak-38 and Harrier was not suitable for the new plane. Instead they created a layout with a single engine, that could turn 95° down, located in the middle of the fuselage, just behind the center of gravity. [Two additional vertical thrust engines] would turn on only during vertical take-off, vertical landing and hovering. Engineers had to stretch body of the aircraft for aerodynamic stability. This is why the Yak-141 is larger than its predecessor, the Yak-38.

The plane was never mass produced

The Yak-141 multi-role fighter did not enter production. The funding for this program ceased in 1991 after a landing accident on the aircraft carrier, when one prototype landed during excessive side wind and was badly damaged. After collapse of the Soviet Union military funding was limited. In 1992 the Yak-141 program was canceled as it happened with many other promising weapon systems. Also by 1995 Russia decommissioned all Kiev class aircraft carriers, this plane was intended for.

The first quotation is largely incorrect. The Yak-141 'engine scheme' is very similar to that of Yak-38, as opposed to Harrier, which had a single engine. The challenge was, apart from control difficulties inherited from Yak-38, to create a supersonic rotating nozzle.

"Two additional vertical thrust engines... ...just behind the center of gravity"  Really?  What are they keeping in the nose, uranium?

@EricHauenstein The phrasing is awful but they meant what they wrote: "Instead they created a layout with a single engine, that could turn 95° down with two additional vertical thrust engines, located in the middle of the fuselage, just behind the center of gravity." The single engine is behind the center of gravity, and it could turn down with the two additional engines.

@RamonSnir That's how I read it as well. They grammar could have been better to indicate the single engine was behind the center of gravity. Because of the way it's vaguely written, it sounds like they could be talking about the forward vertical engines, which would make no sense. I made an editorial edit to clarify what they were saying.

@Machavity I only understood it after reading the Wikipedia article, that phrased it in a more clearer way: "Eventually it was decided that the best arrangement was a single vectoring nozzle located just behind the center of gravity, as well as dedicated vertical thrust jets positioned just behind the cockpit."

@EricHauenstein uranium or tungsten.  Ammo.

Russian YAK-141 aka YAK-41 VTOL (vertical takeoff and landing).
https://en.wikipedia.org/wiki/Yakovlev_Yak-141


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Other than looking cool, what is the reason behind the TSR-2's anhedral wingtips?
Pretty much what the title says. What is the purpose of the TSR-2's anhedral wingtips? I've heard someone say that they use it to counter yawing I think but is this the true reason or is it something else?

Although it looks cool it's one of the few airplanes I wouldn't want to fly. Something about it makes my spider sense tingle.

In the document "TSR2 with Hindsight", a lengthy documentation of TSR2 development by the Royal Air Force Historical Society, the reason given for the wingtip anhedral is the correction of lateral and longitudinal stability. Being a high-wing design, the anhedral was used to lower the sideslip-induced rolling moment.
To quote from TSR2 with hindsight:

At re-freeze, the wing was moved, its planform changed slightly and the tips given anhedral to reconcile lateral and longitudinal stability requirements; a very complex joint rib resulted. The wing already had a centre-line joint but only after great resistance from the weight-saving auditors.

The re-freeze in turn was the result of supersonic wind tunnel testing in 1960:

It was not until March 1960 that supersonic wind-tunnel test results were again available and one effect of the resultant design re-freeze was further refinement of the rear fuselage lines.

Turning down wingtips had been employed before in a high-wing design where re-designing the complete wing had been out of the question: The Heinkel 162. Here, the name for that modification was "Lippisch-Ohren" (Lippisch ears) after the person who had proposed them first for his delta-winged designs.

So if I follow right, they're there to stop the plane from pitching up and down and rolling where they don't want it.

@PeterKämpf  How does wingtip anhedral affect longitudinal stability?

@AdityaSharma I was wondering myself, I guess it affects pitch moments at high AoA. Planform changes are more effective and I think the change was mostly about dihedral effect.

@AdityaSharma: obviously via "longitudinal dihedral"

@AdityaSharma moves the Neutral Point aft for both the longitudinal and vertical axis I would say.

@PeterKämpf the CL-214 water bomber uses similar tips for the opposite purpose,to restore dihedral effect degraded by the fin-like nacelles of the PW-120 turboprops, which had an effect like changing from high wing to mid wing.

Remember the anhedrals are put on the tips of a delta wing  They will help bottom lift there, moving the center of lift aft as AoA increases.  This is essentially how a tail responds to an unwanted pitch-up.  Once the "longitudinal dihedral" does its job setting wing AoA, all forces are in balance for a given airspeed.  If sudden headwind gust pitches the nose up, the anhedralled wing tips adds to the effect of the horizontal stabilizer to prevent the elevator (or pilot) from overpitching the aircraft.

@RobertDiGiovanni This would make sense at supersonic speed, but given that the TSR2 was already stable at subsonic speed, there was no need to increase longitudinal stability at supersonic speed.

@JohnK Intresting. Is there a neutral point for vertical axis? and which direction is considered "aft"?

So, as ever, its a "fix" for a design issue that could not be resolved easily any other way.  Same reason that the F-4 Phantom has turned up wings and turned down tail planes - to fix the poor aero dynamics of the design.

Yes because static stability in the pitch and yaw axis are the same thing, "weathervaning" about the CofG into the airstream, except in pitch you have to include trimming forces to vary the weathervaning target angle. If NP is the point at which all aerodynamic forces are focused, and it needs to be aft of the C of G for the weathervane (static stability) effect to work, the vertical profile of the wing tips, being aft of the C of G on a delta would increase yaw static margin slightly by moving the vertical aerodynamic center aft, and the plan area would increase pitch static margin slightly.


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Do carbon fiber wings need a wing spar?
Considering the structural strength advantages of carbon fiber vs aluminum, would a carbon fiber wing need a wing spar?
I've seen some examples of some carbon fiber wings using some honey comb structure inside the wing, I've also see some use a spar, but don't know what the norm is.
Can the wing be designed with nothing on the inside, the same way a monocoque carbon fuselage is? or is it done this way already? what's the norm?

One must consider not only strength but also stiffness (to avoid flutter).  Fiberglass/epoxy over a foam core is done on model scale.  But not only materials, but also structure.  I would definitely do some homework, as the wing must be sound.

Small carbon-skin wings for race cars often omit a spar.  Sometimes their core isn't even rohacell honeycomb, just styrofoam.
Bigger wings like the A350's need a (carbon) spar, though.  Even larger radio-control model sailplanes with carbon wings can need a spar.

Yes that's actually how modern airliner wings are made, aluminum OR carbon. The entire "wing box" between the leading edge and flap/airleron is the "spar". The upper and lower wing planks, the top and bottom skins, are quite thick because they are the primary tension and compression members that do the job of a spar's top and bottom flange, or "cap" in a normal airplane (where the skin's only job is torsional stiffness and taking air loads).
On an airliner, what are called the front and rear "spars" are just the shear webs forming the front and aft walls of the wing box, which is its own spar. The advantages are less ribs, and great torsional stiffness, critical with long swept wings. To be efficient you need to make the wing in one piece so the entire lower plank is a continuous tension member from tip to tip. Making the wings in sections adds weight because of the need to create attachments that distribute the loads between adjoining members.
In either case you can use carbon in place of the aluminum in the same fashion.
The thick structural planks allow the rib count to be reduced because the plank needs less support. Carbon would allow you do reduce the rib count further as well as carrying the same loads with less weight. Potentially you could get to a rib-less carbon wing, but the weight saving won't be as much as you might think, because you have to make the plank quite thick in that case.

Oh ok yeah, seen so many pictures of airliner's wing and didn't realized the "wing box" was the "spar" itself lol. Yeah makes sense about the ribs not "needed"  with that design.

When you look inside the plank you see rows and rows of lateral stiffeners machined into the plank itself to make it stiff without being too heavy. On an aluminum wing you might see the rib count reduced by half or so.

Wing structures made of an external surface of carbon (or glass) fibers and an internal honeycomb structure are virtually ubiquitous in the rotating-wing world i.e. blades for helicopters and wind turbines. They are also used for the tailplane of several airplanes, not only small GA airplanes but also big ones: the tailplane of Concorde was a good example.

Blade section of a BO-105, the very first helicopter using glass fiber for the skin of the blade and foam for the core, first flight 1967 (figure source).
And the tailplane of Concorde points us toward some cons of this structural solution: The bond between the outer skin and the honeycomb structure is sensitive to manufacture and difficult to repair. Concorde suffered twice the separation in flight of the skin of the rudder, fortunately without any problem.
Anyway for the wing this solution is normally a no-go for another much simpler reason: If the internal volume is taken up by the honeycomb structure then there's no place left for the fuel.

"Is sensible to manufacture"?  Huh?  Did you intend a different word?

@CamilleGoudeseune They may have meant "sensitive" - a common false friend when translating from German

@Bergi: yep correct, thanks. I'm translating from another romance language

Yes, of course they do.
One aspect of a spar is the transmission of shear between the upper and lower caps. If you put unidirectional fibers on the wing skin at the chord of the largest thickness, you will still need a web to keep the two skins apart under load. Or you will end up like Willi Dirks did when he flew the first prototype of the DG-200.
The DG-100 is a standard-class glider derived from the Akaflieg Darmstadt D-37. It used an airfoil with almost 19% relative thickness at the root, but the DG-200 was designed as a racing class glider with flaps and a thinner airfoil of 17%. Willi nevertheless used the same layup for the spar, and with the higher shear caused by the smaller distance between the two caps, the spar collapsed when he flew into a gust at high speed.
If you use a sandwich skin for the wing, you will need to either put the unidirectional layers on the inside of the skin sandwich or to put a gap in the sandwich to transfer the shear stress between web and wing skin. Essentially, this will make the skin at the maximum thickness into spar caps and the web will complete the spar. If you use two webs spaced a bit apart, the resulting structure will form a torsion box - spar combination.


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Why does the USAF still use the C-17 Globemaster III?
The C-17 had to overcome flight-control problems, wings that were unable to carry their designed maximum load, automation growing pains, and a crew size reduced to just two pilots and a loadmaster so I don't understand why the USAF still uses C-17s and not just KC-10s or invest in the A-400M Atlas instead.

You seem to be ignoring the fact that the C-17 was fully operational a good 20 years before the A400M. The A400M doesn’t seem to have been an option when the USAF was acquiring the C-17, and why would they throw away all that investment after it was acquired and all the bugs had been worked out? The A400M had plenty of problems of its own.

Problems were fixed long ago and a C17 is twice as big as an A400m. KC10 are out of production.

The C-17 still has major software problems

why the USAF still uses C-17s and not just KC-10s There is very little overlap in what those aircraft can do. You can't land a DC-10 on unpaved runways. Nor does it have roll on-roll off for cargo.

Just to add something to the other answers.
The C-17 has been born as an answer to a Request For Proposal (RFP) from the USAF. "By 1980, the USAF had a large fleet of aging C-141 Starlifter cargo aircraft. Compounding matters, increased strategic airlift capabilities was needed to fulfill its rapid-deployment airlift requirements. The USAF set mission requirements and released a request for proposals (RFP) for C-X in October 1980"

why the USAF... not just invest in the A-400M Atlas instead?

Regarding the A400M: in the US, when a new military aircraft (or military aerospatial equipment) is requested, only the relevant national aerospace companies (if any) are normally (but not exclusively) allowed to submit a proposal. This was true for the C-X program and that obviously excluded any non-US design from the competition: no matter how good the A400M might have been, it was not an US design. Plus, the first idea for what would have become the A400M dates to the 1982, i.e. much later than the original RFP from the USAF.

why the USAF still uses C-17s and not just KC-10s?

Regarding the KC-10: "Alternative proposals were pursued to fill airlift needs after the C-X contest. These were lengthening of C-141As into C-141Bs, ordering more C-5s, continued purchases of KC-10s, and expansion of the Civil Reserve Air Fleet". So the idea to buy more KC-10 was actually there. Why it wasn't pursued further is not specified but I suppose it was a mix of political and technical aspects. I can't say anything about the possible political reasons but from a technical point of view the lack of a rear ramp and a conventional tailplane is for sure a big no-go for a cargo airplane.

Quoted text from Wikipedia

"only the relevant national aerospace companies are allowed to submit a proposal"  Eight non-US governments bought the C-17, and four of those have access to domestic heavy aircraft manufacturing.

@AnonymousPhysicist: and what does this have to do with the design phase? The final product can be sold to whichever ally you want but it must be made in US (or whatever country issue the RFP). Please note that also the "winning answer" mentions this aspects.

"Why it wasn't pursued further ..." (meaning the purchase of KC-10s): I guess a mix of production ending of the underlying aircraft type (the DC-10) by the end of the 80s and strong competition from cargo airlines.

"only the relevant national aerospace companies are allowed to submit a proposal" That may be true for the USA, Russia, China, but not for smaller countries.

@MikeB: obviously if a country has no aerospace industry it cannot issue an RFP for its non-existing aerospace industry... do we really have to play these semantic games?

@DohnJoe: also good points

@sophit Brazil has a relevant aerospace industry, but in their RFPs regulary participate other country industries (France, US, even Russia ...)

@sophit "Regarding the A400M: As usual in the military world, when a new aircraft or equipment is requested, only the relevant national aerospace companies (if any) are normally allowed to submit a proposal. "
The "military world" does not end with the "US military world" (although the military industrial complex influence is projected well beyond the US https://en.wikipedia.org/wiki/Military%E2%80%93industrial_complex )

@EarlGrey: edit approved and comments deleted

The C-17 is made by a US company, Boeing. The A-400 is not.
KC-10 is long out of production.
Every aircraft in the history of ever had growing pains. If we bailed at the first "problem", there would be nothing in the air.
For "growing pains", the A-400M:

The aircraft has experienced technical problems, schedule delays, and cost overruns since the programme was launched in 2003.

From https://www.janes.com/defence-news/news-detail/airbus-takes-another-a400m-charge

This answer covers some significant differences in the aircraft. The key difference there is maximum cargo weight.
The A-400M has a maximum cargo of 81,600 lb. The C-17 has a maximum cargo of 175,000 lb. Thus, the C-17 can carry more than double the weight (like an M1 Abrams tank). Remember, the US military is often projecting power half a world away. You need to be able to move that heavy hardware at a moment's notice. The downside is that makes for a shorter range and longer runway requirements, as well as more fuel consumption.
This is also why the USAF still operates the even larger C-5 Galaxy (maximum cargo of 281,001 lbs).
The A-400M is designed more for range, versatility and economy (turboprops are more efficient). It can move troops and light armor (humvees, etc). In other words, they serve different roles.

The USAF already has a fleet of C-17 aircraft. In FY21 they reported a mission capability rate of 80%. This is higher than the C-130H/J and C-5M which are the other main transport aircraft it operates (and even slightly better than the KC-10). Compare this to the A400M which is at... 35%? 70%? So the USAF already has a fleet and it performs the missions they need it to do. What incentive do they have to replace it?
Although it has a similar cargo weight capacity, the KC-10 is a very different aircraft. The fleet is only a quarter the size of the C-17 and it's way too late to produce more of them. It also lacks a very important feature that the other military airlift aircraft has, which is a built-in ramp to allow cargo to be loaded directly into the aircraft. The KC-10 requires large support equipment to load and unload cargo. The KC-10 is also not designed for operating from short or unpaved fields.
The A400M has the important design features for a military airlift (ramp loading, short/unimproved field capability), but it has less than half the cargo capacity. And this isn't just a matter of needing to buy more of them, since heavy vehicles like tanks are beyond the capacity of the A400M. The smaller size means it also has less range than the C-17. And lastly a more minor point is that the USAF uses boom refueling and would likely require this capability, so they wouldn't be ordering the A400M just off-the-shelf with the probe system currently in production.
Another argument for how these aircraft fill different roles is that the UK RAF currently operates both the C-17 and A400M, finding a use case for both aircraft. The French even borrowed a couple of their C-17s when they needed to move some heavy vehicles.
The only real alternative to the C-17 is a completely new aircraft, which will certainly come along eventually as the C-17 fleet ages. But this is extremely expensive and has all the risks that come with a new program.
And as has been pointed out, no aircraft program of this scale is ever without its issues, including the A400M. The only hull loss of a C-17 in almost 30 years of service was due to pilot error. The A400M has been in service just over 10 years and already had a fatal crash due to a software/maintenance issue. That's not say that the A400M has more issues: both are well in the realm of "small numbers" so it's hard to conclude much from this other than to say that they have both been fairly successful in their respective missions.

KC-10 went out of production before the C-17 entered production. It's also a completely different class of aircraft and can't carry the large size loads the C-17 can and land them on unimproved short runways
A400M is much smaller, has shorter range, and didn't exist when the C-17 fleet was built. It's not a C-17 competitor, it's a C-130 competitor (and countries that buy it tend to buy it to replace their older C-130 models)

IOW, you're comparing apples and oranges. May as well ask why the army uses Hummers when they could buy Mustangs instead.

I like the analogy and thank you for this helpful answer


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Can anyone explain the reflex wings used in powered paragliding?
There are some paramotoring wings presumably designed for higher speeds. The method is to allow a portion of the wing near trailing edge to flutter in high speed, rendering it unaccountable for development of lift, but still accountable for drag (in my opinion). I believe is called reflex because the trailing edge will flex up in high speed.
The speed gain is minimal though. These wings fly at around 40km/h with stall at 24km/h and the reflex promises 60km/h, while the same wing trimmed non-reflex achieves 55km/h.
If there is anyone with piloting experience on those wings, please explain the reflex from your perspective.

I'm haven't flown them but am very familiar with them and the reflex wing controversy because I was interested in getting into the sport (I normally fly power and gliders).
A reflex wing when in "reflex mode" offloads the aft part of the wing and shifts the pressure distribution forward.  Effectively, the rear part of the wing is being allowed to "trail", or partly trail, up and not do any (or much less) work.  It's a bit similar to gliders with flaps that have a reflex setting, where the flap is angled trailing edge up a few degrees for high speed penetration flying, which off loads the trailing edge and moves the wing's pressure distribution forward and makes it behave as if it had a narrower chord.
It's not related to reflexing flying wing aircraft trailing edges for trimming forces, where the trailing edge provides downforce in place of the absent tail to trim the body to a given AOA; that simply can't work and isn't necessary in a paraglider.
The effect of reflex is to somewhat reduce the effective wing area, increase the effective aspect ratio, and reduce induced drag in the process.  A 26 sqm wing becomes effectively a 20 sqm wing (I'm just picking numbers here) with a narrower chord and what amounts to some loose fabric (the unloaded trailing edge) trailing behind it.  Because the TE is unloaded, you can't use the normal trailing edge drag brake steering and have to use "tip steering" controls.
The different pressure distribution increases the inflation pressure at the front of the wing and makes it more collapse resistant, and this is one of the reflex wing's selling features besides slightly higher cruising speed.  The downside is that when they do collapse, they do so violently with way more altitude loss and if you are close to the ground your wing will pretty much fling you into the dirt like a rag doll if you get a collapse.  I don't believe there are any current wings that can meet EN cert requirements while in reflex mode.  This means if you value your life you don't use reflex below a recoverable altitude, say 500 ft.
Having researched it a lot myself and noted the same thing you did, only a 5 kmph benefit, I'm not keen on reflex myself, especially after watching videos of how they behave when they collapse.

+1 for explaining that "reflex" in flying wings is completely different.

Thanks. There is a... somewhat colourful character, well known in the paramotor business, by the name of Schanze, who rants on about how dangerous reflex wings are, and I tend to agree, but he goes on about how they can't work because you "can't push on a string", as if it was a reflexed flying wing.  He misunderstands what is going on.

You can't reduce induced drag if the mass per span stays constant, unless you improve the span wise lift distribution (something I don't expect a reflex wing to do). My guess is that a reflex wing at low lift coefficients shifts the stagnation point a bit down, allowing a more favorable pressure distribution on the bottom of the wing.

That may explain the increased internal pressure and collapse resistance.

@PeterKampf Actually, can't induced drag be reduced by lowering AoA and going slightly faster?  A parasail is a thin undercambered wing.  It is great that the dangers of lowering AoA are pointed out.  Some strange things start to happen with circulation under the wing at lower AoA. Dynamic pressure is needed there to hold it's shape.  At lower AoA, it is more prone to collapse.

Three definitions first (simply): CoW (Centre of Weight) and CoP (Centre of Pressure). These two govern the aircraft (any) while flying (lets forget about thrust and drag now - the other two). AoA (Angle of Attack) is the anlge between the leading edge and the relative airflow.
In case of airplanes CoW and CoP are close together. For a Paraglider, CoW is significantly below CoP (normal flying). So it is possible that when AoA changes, CoW "moves out" from below CoP. The natural behaviour of all constructs in the air that CoW "wants to be" below CoP.
The difference between normal and reflex profiles is what happens when AoA change.
Let's say you hit an uplift stream (turbulent air going upwards).
This will increase AoA.
In case of "normal" profile, this will result CoP move FORWARD. But, hence CoP "wants to" be over CoW, the whole glider will "slide backwards" a bit. This will increase the AoA even further, which will move CoP even forwards etc etc. It would cause the glider to stall, but by the time it would stall the pilot's pendulum will stabilize the system.
So in rough air, normal gliders "move a lot".
In case of reflex profile, CoP moves BACKWARDS instead of forwards, moving the glider itself forward above the pilot, which decreases AoA, so the two effects negate each other. That is why reflex gliders are more easy on rough air.
The fact that they are faster has nothing to do with profile.
Reflex gliders have different line distribution and different sizing, but these are independent "changes". Okay maybe not completely independent - a reflex glider, when fully trimmed in, has "almost like" a normal profile - to easy the job with takeoff and landing. But this means that they will get their relfex profile, once they are trimmed out, so weight is distributed towards the front lines. But the more full-bloodied reflex glider you get, the less this is a compromise.
IF you overload a normal glider, it will be fast as any relfex glider.
Also worth to note that reflex is more difficult to open once closed up for any reason. But they fold much harder.

"So it is possible that when AoA changes, CoW "moves out" from below CoP. The natural behaviour of all constructs in the air that CoW "wants to be" below CoP." Oh look, it's the pendulum fallacy! Please see here.

@AEhere supports Monica - Well, you can object to the "wants to" construction, but the end result is the same as if it does "want to".  Imagine a heavy, streamlined weight fixed in position a ridiculously long distance below a wing by a thin but rigid rod.  That wing will tend to be roll-stable even with no dihedral, and pitch-stable even with no horizontal tail.  The reason has to do with the roll torque created by sideforce during sideslip following an accidental bank, and the pitch torque created by increased drag due to increased airspeed following an accidental dive.

@AEhere supports Monica - In both cases the aerodynamic forces act, very loosely speaking, near the center of surface area of the aircraft, far above the CG, thus creating a stabilizing roll or pitch torque.

@AEhere supports Monica - Naturally, all this is important to understanding paraglider stability dynamics, where the pilot is fixed in place far below the wing.  Positive roll stability results, despite the anhedral wing shape.  Positive pitch stability results as well.  Now, what if the paraglider wing were sprayed shellac to freeze the fabric in the familiar shape that we normally see in flight, but the lines were removed, and the pilot rode on a scaffold that fixed him in place high ABOVE the wing.  Would that aircraft have any hands-off stability in pitch or roll?

@AEhere supports Monica - consider also a toy weight-shift-controlled radio-controlled "trike" powered hang glider with Rogallo delta wing-- the aircraft gets a boost in pitch and roll stability due to the heavy "trike" unit with wheels, electric motor, battery, and servos held in a fixed in position via the servo linkage, well below the wing.  It could never under any circumstances be kept in sustained inverted flight.  Just as a paraglider could not, even if we replaced the lines with rigid struts, and "shellacked" the wing to fix it in its normal positively-loaded shape.

AEhere supports Monica - so in actual practice, it seems there is something true about the idea of a stabilizing "pendulum effect", even though that terminology may tend to give an oversimplified impression of exactly what is going on.  Note also that in the case of roll dynamics, it is not necessary to have a true "sideforce" to create a stabilizing roll torque during a sideslip via the "pendulum effect"-- any component of the aircraft's drag force that acts sideways to the aircraft's longitudinal axis, and above the CG, will suffice.

I thought AoA was angle between chord and relative airflow. Saying "the anlge between the leading edge and the relative airflow" doesn't really make sense to me as the leading edge doesn't have an angle so to speak.

Fluttering is usually undesirable because it can lead to tearing and usually increases drag, which tends to lower airspeed.  A raised trailing edge is similar in function to an elevator, increasing drag and limiting airspeed.
The concept may be similar to that employed by the "Suzanne" record breaking hand thrown paper airplane design, that had a slightly upraised trailing edge.
Thrown (for distance) free flight paper airplanes are deceptively challenging because of their extremely high speed range.  Discus Launched Gliders are trimmed to be staticly unstable$^1$ when launched, but can be re-trimmed by radio control once they reach height. No such luck with free flight.
John Collins ingeniously designed a "reflex" or flexible trim mechanism that would lay flat under higher airspeed dynamic pressure, but "pop up" ay lower airspeed.
On the world record throw by quarterback Joe Ayoob, the plane did indeed fly out straight, lost airspeed, and stalled, but recovered in the dive and flew on to a world record.
Essentially, at higher airspeeds, reflexing reduces static stability and drag$^2$, allowing the paraglider to go faster without changing control input.
The reflex concept is not without some foundation.
$^1$ going faster lowers Angle of Attack
$^2$ light "luffing" may be noted


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Why do airports have directional runways instead of a large area to operate in any direction?
Instead of a few runways in a few directions, why not have a large square block of asphalt so that planes can take off and land in any direction? Where and on what direction planes land and take off will still be determined by ATC, but these decisions are then not bound by the choice of just a few runways. So, there will only be virtual runways and taxiways instead of real physical ones.

It looks... expensive.

What problem is this trying to solve?

Theoretically, it could be possible a single asphalt block runway. But this would raise numerous safety and security issues in modern aviation. Perhaps too many to be even considered as a feasible alternative.

Danger, chaos, confusion and no clear solution to a problem, whats not to like about this idea?

Hydrobases kind of have a single big block (a lake) that is used as a runway.

Even when nature provides us with such a nice flat area, the human tendency is to divide it up into official runways: https://en.wikipedia.org/wiki/Rogers_Dry_Lake

For the same reason that streets have painted lane markings.

"Why build roads when you could pave the entire planet, so cars can just drive anywhere. GPS could still send you into the general direction. Virtual roads!"

I would offer a circular "runfield" as most symmetric solution. It could be painted in concentric and radial patterns and could be made a bit dome-shaped in order to manage the rainwater (somehow). Well, it still won't be any better than the traditional runway and will still be expensive.

@fraxinus Like the one is this question?

@PerlDuck Not really. The OP here offers a square, I "optimized" his square by cutting edges to make it symmetric and equally "long" in all directions. What you link is a traditional runway, just bent "a little" to meet both ends together. There is no tarmac in the middle and that makes it a little bit more (but still not exactly) possible.

Where do you propose the ILS goes?  On a lazy susan?

Rather than a big square block, how about a circular runway?

I'm glad someone fixed the title.

@NumLock - roads are not a good analogy to this since first, covering the planet with roads is impossible, while airports are a finite (though very large) area. And secondly, major airports have a single controlling entity that could control the angle of arrivals/departures while roads have no central controller. Still doesn't make it practical, but it's not comparable to doing it to roads.

Instead of a huge runway with n^2 resource requirements, why not just make the runway rotate? :-D

@Michael I wouldn't say it is less expensive, but still done in practice: https://en.wikipedia.org/wiki/Aircraft_carrier. Added bonus: you can run the whole runway at some speed in some direction in order to make a favorable artificial wind.

Presuming everything is automated, this makes sense on the surface, except even humans have no problem as it is. The entire plot would still be made by one, ten-foot-wide, lane at a time creeping inches per minute. And after the first one's done, they say if we just had one more going the other way, it'd be fine. Custom-Configured Cat Paver Helps Contractor Meet Runway Specs – YouTube

Where do the runway lights go?

The typical World War II "A" runway shape solves this issue, without covering a huge amount of area with concrete, etc. Even so, that configuration has fallen out of use in modern airports.
Ultimately, this is a solution to a problem that doesn't really exist and a problem which has a more efficient solution that was already widely used.
With respect to your particular question - to some degree, this is exactly how original grass fields were treated. But no matter how convenient, there's always a need for consistency and predictability and you can see how that has developed to what we have now. Taxiing a heavy airliner isn't a trivial task - doing so with no markings or defined route sounds terrifying.
Additional practical issues of your large slab of concrete vs. a defined runway:

No ILS
No runway lighting
No runway markings
No threshold, no touchdown markers
Far more difficult to precisely define runway length
Very very difficult to accurately describe a particular location and route

To be honest, the list goes on and on and I'd struggle to imagine commercial ops being able to operate like this safely and efficiently.

A large slab would also be much harder to check for debris, and much harder to drain.  By the time you'd painted enough markings on to keep fast-moving things away from other things, you might as well let the grass grow in between the defined zones

Related:https://aviation.stackexchange.com/questions/1898/could-you-land-a-large-airplane-on-short-circular-runways

No ILS, but 36 RNAV (or even GLS) approaches would cost less than two ILS approaches anyway. Whether ATC could handle that is another matter.

I found the class A airfields (https://en.wikipedia.org/wiki/Class_A_airfield) interesting, it might be worth mentioning. There is a trade-off between any-direction landing and dealing with some crosswind.

Most of the practical issues noted have a common and relatively straightforward solution; embedded, configurable, dynamically controlled lighting.  You can paint taxiways, runway markings, touchdown markers, entire runways (dialed to any desired length that will fit within the area), exclusion zones, and so on anywhere you need, as and when needed.  Would be incredibly expensive, but certainly doable from a technological standpoint.

@aroth Once you've painted something, you now have a defined runway and may aswell have just started with that. I also don't think it's that easy or straightforward - even for something like lighting.

Adding to the list: snow removal would be a major concern in my location. A single standard runway can require a large maintenance budget, crew, and specialized equipment just for snow removal.

@aroth Uhhhh where is this dynamic runway lighting supposed to be installed?

@Dan I should have been more clear.  Where I said "paint" I meant "display, using the dynamically controllable lights embedded throughout the concrete surface".  Not to literally put actual paint on the thing. DKNguyen - all throughout the surface of the concrete slab.  It will be exorbitantly expensive!  And since the embedded lights must be clearly visible in broad daylight, the energy usage will be ridiculous, too. But it's technically within the realm of what could be done. :)

We have runways for the same reasons that we have roads:

All traffic needs to move in the same direction, in visually easily identifiable lanes. Channeling the traffic improves safety and efficiency.
All of the area outside the lanes need not be paved. Increases cost efficiency.
Traffic lanes and runways can be constructed for rainwater run-off, flat fields cannot.

Image source

The 1.5% grade for drainage, that is the severity of grade at which railroad freight trains will be seriously considering adding  "helper" engines to assist on the grade. These days they mostly do it for dynamic braking downhill, because locomotives with AC motors and VFD have the low end power to climb them unassisted.

@Harper-ReinstateMonica I think you misunderstood the picture: the planes don't move from left to right but from front to rear and don't have to get over a 1.5% gradient. The "bump" is between the left and right main landing gear. ... or maybe I misunderstood your comment.

Also, aircraft are designed with a substantially higher thrust/tractive effort to weight ratio than trains. 2% slope lengthways seems to be the maximum for commercial runways.

Land costs money and paving it costs money. For a typical commercial airport like Grand Rapids Ford Int'l, your proposed runway would be about 3 square miles, which could easily cost about \$40,000,000 for the land, and another \$20,000,000 to pave it.
Your proposal would have very little benefit; right now, Grand Rapids Ford Int'l has two main runways at a 90-degree angle to each other, meaning that airplanes may have to land at up to a 45 degree angle to the wind. That's almost never a problem.
So your suggestion would cost about $60,000,000 and provide essentially no benefit.

The land costs are pretty well sunk anyway - there aren't any airports that don't own the land between the runways & taxiways, are there? The paving cost is 100% valid. Oh, and don't forget the expense of maintaining that massive slab o'pavement instead of just "thin" ribbons of pavement.

@FreeMan Well, generally, building the proposed mega-runway would require either buying some new land, or demolishing lots of existing buildings. In the case of the Grand Rapids airport, it would require demolishing the existing terminal, buying lots of neighboring real estate, demolishing those buildings, and building a new terminal on some of the leftover land.

Think of the drains, too! With hydrocarbon-contaminated runnoff. And on a surface where coeffieient of friction is critical, over three square miles!

also, the maintenance... the paved area needs to be maintained as well, the more there is to maintain, the more effort and cost there is

@FreeMan Most airports use that space to park aircraft and other vehicles, build terminals and cargo handling facilities, office space, warehouses, etc. etc.. You'd need to buy more land to relocate all of that, and have longer distances to cover for the aircraft and other vehicles to get to and from all those new buildings.

@FreeMan and a lot of small municpal/regional airports tend to lease the land in between out to farmers for growing crops or hay as well.This helps them recoup some of the cost of the land they have to own, and reduce their maintenance cost as they don't need to maintain that area.

@FreeMan I feel the need to mention Narita …

@FreeMan In what sense is the land costs sunk? If you are claiming that airports could do this with land they already have, then shouldn't just just assert that? I get the impression you don't fully grasp what "sunk cost" means.

Airfields used to have that, big fields where the pilots could line up and takeoff or land into the best headwind.  Might be okay if the field was small, say up to 3000 feet by 3000 feet.
But, where do you set up ground based instruments that help with poor visibility landings (low ceilings due to fog, rain, etc)? Radio antennas, lights, etc. Having it hard installed/wired is the most feasible and durable.
And then once the runways get longer, the sheer size becomes an issue, along with development that seems to spring up around airports over time.
So the runways are fixed, and we learn how to handle the plane in crosswinds.

Consider that a large commercial airport has 12000ft long 200ft wide runways (about 3.7Km x 60m) not including blast pads and overrun areas. (cheaper pavement as they don't take weight in normal operations and can be inspected/repaired after emergency use) A square would cover 14 square Km or 5.2 sq miles or 3300acres. All at a thickness that can support a hard landing in a fully loaded 747.(about 18-24inches thick of med-high strength concrete with some re-enforcing steel)

A large number of answers so far, so I won't cover budgets. But I didn't notice any mention the approach and departure paths. These flight paths must be kept clear of obstacles above a certain geometric plane and the common folk love to purchase land near an airport or race track and then complain about noise so flight paths are designed to minimize trouble with neighbors, also there is risk with a busy airport and aircraft traveling off the ends of the runway. Jet blast also can not be directed at parked planes.
Then you have issues with design of instrument flight rules approaches and departures.

Not to mention that in the less flat parts of the world, those obstacles are often mountains.

@jamesqf 'tis but a hill!

In the old days many "taildragger" planes were literally that: the tail dragged on a non-steerable skid. The skid didn't work well for keeping the plane straight on hard pavement.  Also some of these planes didn't handle crosswinds well even on grass.  So planes took off and landed on wide-open grass fields where pilots were free to take off and land directly into the wind.
Now, almost no planes have tailskids, and most have tricycle gear configurations, and crosswinds are much less of an issue than they used to be.  Two runways oriented at near 90 degrees provide an adequate reduction in the crosswind component for most situations.  Modern planes, especially airliners, have much heavier "wheel loadings" than the old antiques, and require heavily reinforced pavement.  It wouldn't be cost-effective to pave an entire square field or other similar shape in this manner.
Also, as another answer has noted, modern instrument landings and takeoffs need to have elaborate systems to transmit radio beams, along with elaborate lighting systems.  If you have such a system, it makes sense to have a runway aligned with it-- though granted, the need for precision would be reduced, as far as maintaining a target course, if you were aiming for a gigantic paved area rather than a runway, and had clearance to land anywhere on it. But if you drifted too far off the intended course, you'd also lose the beam that is giving you vertical guidance, unless the beam is somehow designed to cover a very broad area.  Maybe as GPS systems continue to play an ever-more prominent role in instrument landings, the radio beam issue will become less of an obstacle to your idea.
But there are other issues as well.  Paving an enormous field also creates environmental problems-- where does the run-off water go?  And  so on and so forth.  On the whole, there is little upside to your idea in the context of modern airline operations, and lots of downside.

Previous posters have given reasons not to use your proposal of designing airports as huge square slabs of concrete with no discernible runway for large airports. I don’t think this would be feasible for small airports, either. I have yet to land at a GA airport that took up a perfectly square plot of land. Most airports are long and narrow. The exception is the few airports with runways set up in roughly “A” or triangle shapes. Or, airports set up with two runways at a roughly 90° angle. It is easier to find and buy/procure land for an airport that is longer than it is wide. And, length is important to have distance for takeoff and landing. A 4000 by 1000 foot area would be more useful than a 2000 by 2000 foot area. Plus, you would have more options available geographically. Then, there is also the issue of where to put the necessary facilities like towers, fuel, hangars, FBO, etc. Placing these along one edge of a square limits the use of that edge or axis for flight operations. If you place the facilities in the center, you limit their access by land based traffic.

Well, there was Black Rock Interstellar, back in the old days (pre-2000 or so), where you could land & take off pretty much any which way.  But I understand they've gotten much more officious these days.

If you place the facilities in the center, you limit their access by land based traffic. Obviously you dig a tunnel underground for routine land vehicle access, because clearly money is no object!  But seriously, good point about logistics.  You can't just have aircraft land anywhere at any angle if you have terminal buildings in the middle of the square.  Although along one side probably makes the most sense.  Still lots of room to land beside the terminal, somewhere else along this vast wide area.  But then you might have to taxi all the way to/from the far end.

Primarily costs.  Paved runways cost around $1-2 million per 1000 ft, depending on construction and features.  An airport like KATL probably has a quarter of a billion USD invested in the runways, taxiways and reinforced concrete infrastructure alone.  In addition if you did make, say, a 2 x 2 mile concrete pad to the standards of a runway, most of this structure would never really get used, making it a colossal waste of money.
Given the high costs and other requirements, it’s easier just to lay out single runways with connecting taxiways and parking aprons.

Something i havent seen in any other answers, ATC logistics. ATC is already considered one of the worlds most stressful jobs, and planes can only land 2 ways on a particular segment of land (and taxi). What you are proposing is a situation where ATCs have to direct planes to land in a specific spot wothout deviation or they risk a collision, whilst not being able to clearly see half the airspace in due to the sheer distance and lack of distance markings.
This is not including the other logistical problems with taxing due to the lack of taxi ways and more importantly hold points. Without taxi ways and hold points getting taxiing aircraft off one ‘runway’ but not on another and other movement procedures would become much harder. Current systems still use taxiways labeled with a small sign on the grass between runways/taxiways etc. What you are proposing is a system with no markings, or 360 sets of markings so they can rotate the active ‘runways’ directly with the wind.
Oversll the confusion and stress as well as the added cost and logistical problems mentioned in other answers would not be worth the small gain from such an airport.

If planes could land from any direction, where could you put the tower?  It is higher than the runways, and is an essential part of controlling the movement of planes on ground and in the final stages of take-off, landing, touch-and-go, and other operations.
Other items around an airport like to have elevation, such as radar installations.  These would need to respect the flight paths from anywhere to anywhere on the landing plane.
By having specific approach and take-off routes, the region surrounding the runway can be built upon.  Otherwise there would be onerous restrictions on development well outside the mega-slab runway.


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Can you do a IFR flight in a uncontrolled space in a helicopter?
Can you do a IFR flight from point A to point B (both within an uncontrolled airspace) in IMC in a helicopter?
Would there be a minimum altitude?

IFR in uncontrolled airspace (classes F and G) does not require a clearance. You still have to fly whatever the legal minimum altitudes are. In the US, that's 1000 feet above the tallest obstacle within 4 nautical miles (2000 feet in designated mountainous areas). Since controlled airspace starts 1200 AGL at the highest through the vast majority of the lower 48 and obstructions aren't even depicted on sectionals if they're shorter 200 feet AGL, this makes uncontrolled IFR flight impossible as a practical matter. The maximum altitude to stay in class G is less than the minimum altitudes for obstacle clearance throughout much of the US, and they are very, very close to each other everywhere else.

The defining feature of uncontrolled airspace is that you are allowed to fly IFR without a clearance. It doesn't matter what type of aircraft you're using; in uncontrolled airspace your scenario is legal.
Minimum altitude depends on the local regulations; in the usa that's faa-regulations 14 CFR 91.177, which applies equally in controlled and in uncontrolled airspace. Of course in the USA you would be extremely hard-pressed to comply with 91.177 while remaining entirely within uncontrolled airspace.
Note that by "uncontrolled airspace" we mean specifically Class F and Class G airspace. VFR pilots often refer to Class E airspace as "uncontrolled" because there is no ATC communication or clearance requirement for VFR aircraft in Class E, but this is not the case for IFR aircraft.

I see, and what would be the minimum visibility in USA to do a flight like this?

In-flight, minimum visibility is 0SM. For takeoff from a civil airport, 1/2SM per 91.175(f)(2)(iii), unless otherwise specified for that airport. But that's for an air carrier or similar operation... for a non-air-carrier operation, there are no prescribed takeoff minimums (i.e. a zero-zero takeoff is legal).

Oh wow no minimum for a non carrier. But would that be even safe? What would you do if for example right after you takeoff (let's say at 100 feet), you lose engine power and you have to put it down with such low visibility?

When it comes to regulations, what is legal may not be safe (and to a far lesser extent, what is safe may not be legal).

yeah that's right.


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Twin engine or single engine with parachute, which is safer?
Statistically speaking, if we compare two airplanes: a twin engine (like the Beechcraft Baron) VS a single engine with a parachute (like the Cirrus SR22), if we add up all posible emergency events(engine failure, out of fuel, loss of control, etc) and their probability, on which aircraft do you have more odds to survive?

Don't have data handy but traditionally, singles have always been statistically safer than piston twins, because the single engine performance of a piston twin that is loaded is so bad. Hence the old joke, "the remaining engine takes you directly to the scene of the crash".  Richard Collins in FLYING magazine used to write about this all the time. A parachute would raise SE safety further.

@JohnK  it is safe to say a single is also safer in the case of a jet engine?. Let's say the Cirrsu Vision Jet VS a Embraer Phenom 100

If we add data for twin-engine jets, the stats probably tip toward the twins, just because there are so many Boeings & Airbus & Gulfstreams & Lears & whatnot flying around with safety stats well above any GA, single or twin. Basically zero single engine anythings are maintained & operated at Part 121 standards, and not many at Part 135.

@RalphJ yes once the single engine climb exceeds, say, 500 fpm or so in marginal conditions (loaded, hot) the risks drop off sharply. So turboprops and jets don't apply to this when I say twins because they can generally get away when one quits during takeoff. The piston twin might give you 2-300 fpm at gross wt, but that's only if you fly it perfectly, on blue line, 5 deg into the live engine, all of that. At 5000 ft density alt, gross wt, the good engine is more or less just flattening your glide.

@Gabe no because twin jets generally have oodles of reserve power and can climb decently single engine. Take away half the thrust of a jet, and it still might climb at 1000-2000 fpm. Take away half the power of a turboprop, you still get 500-1000 fpm, Take away half the power of a Piper Seminole, when loaded at gross, and it's barely enough for level flight plus enough for a couple hundred fpm of climb if you fly it properly. In the case of a jet, I'd go with the Phenom unless its engine out performance was really bad.

@RalphJ yeah that's a good point, the Part they operate in definitely has an impact. There is actually a company called "Verijet" which charters the  Cirrus VisionJet (exclusively for that matter), don't know if you've heard about it. Yeah, would be interesting to make a comparison with the counterpart flying under the same Part.


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Has there ever been an accident caused by a laser pointer strike?
As seen in the FAA's Laser Incidents by Year data, the number of laser strike incidents has increased over the years.
I wonder if there has been a strike serious enough to have caused an accident* in the USA or any other country?
* The definition of accident by ICAO is: A person is fatally or seriously injured or the aircraft sustains damage or structural failure or is missing/inaccessible

I believe this one, in the US, would count as an accident, according to ICAO definition:

Partial blindness requiring medical treatment
Nov 20, 2022
Zenith CH 750 Cruzer
Landing at Arlington Airport, Snohomish County, WA

According to records filed in the case, at about 5:00 PM on November 20,2022, [...] points a blue laser pointer at the cockpit of a two-person personal aircraft. The plane was on a track to land at Arlington Airport. The pilot’s eyes were damaged by the laser beam, and he was unable to see his instrument panel. He was able to switch to the backlight on the instrument screen which allowed him to safely land the aircraft. The pilot needed medical treatment for his eye injury.

However there was no investigation by NTSB, and the police report isn't public. This laser strike was neither registered by FAA as an accident nor an incident.
Was the laser strike able to cause an accident? According to the pilot:

If I had been flying an older aircraft without the instrument visual aids, I likely would not have survived.

and according to the judge, to the defendant:

You almost killed this man.

A second laser strike, less harmful, was attempted at another aircraft flown by a student, the instructor was able to take pictures:

(Source)
Based on news:

Pilot blindly lands plane after eyes are damaged by laser aimed into cockpit

Snohomish County man who aimed laser at two small planes sentenced to prison


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What's the precision difference between ILS and GPS(LNAV/VNAV)?
What is the precision/accuracy difference between both GPS (LNAV/VNAV) and ILS?

Related: How precise is an ILS (Instrument landing system)?

Do you mean the specification of the approach or the operation of technology? Which of precision and accuracy do you want? Precision is repeatability with low variation and accuracy is how close you get to the real target. This is potentially four different answers.

I mean the operation of technology. Like how precise is one vs the other.

Corrected GPS technology is far more accurate and precise.
Focusing strictly on the technical operation of ILS vs GBAS GPS, a study directly compared the two long ago when GPS was being considered for use in precision approaches. GAST-D was an earlier name for what became GBAS (ground based augmentation system), which is the most accurate of the mainstream GPS approaches.
An A320 was equipped with a compliant ILS receiver and the new GPS equipment, linked to the ground station correction equipment. Six approaches were then flown against a reference localizer and approach. This chart shows the vertical GPS (red) deviations at less than 1m while ILS (blue) deviations were as much as 5m high and 8m low. Horizontal deviation for GPS was less than 0.5m while ILS was as much as 10m. The reference contains the technicalities of the methodology.

This is very interesting, but I'm not sure it answers the question. Of course GLS (and also LPV) will be at least similarly accurate as ILS, since both are augmented precision approaches. But the questions asked about non-precision LNAV/VNAV, which uses GPS without augmentation for lateral navigation and (in case of Baro-VNAV) barometric altimeter for vertical navigation. That's probably far less precise.

GPS + GBAS is an entirely different animal than GPS by itself.  LNAV/VNAV uses an IRS (or multiple) that receives GPS updates, but if we're just talking about LNAV/VNAV per se, you're into the realm of ANP values, which are typically a couple hundredths of a NM.  Good accuracy, but not at all the same as what's achievable with GBAS.

@Pilothead Thanks a lot for that info. As @ Ralph and @ Bianfable said, GPS+GBAS is a different animal though. I'm interested in knowing the difference between ILS and GPS by itself though.

OK, but I don't think anyone even makes uncorrected GPS equipment for aviation and you'd be nuts to turn it off just because you fly an approach that doesn't require it.  WAAS at least is used on LNAV/VNAV, so the difference with LPV is really only in the specification of the approach corridor which was excluded from the question. In any case I'll leave the answer.

"I don't think anyone even makes uncorrected GPS equipment for aviation" -> that may be true for newly designed aircraft, but most airliners that are flying today have neither SBAS nor GBAS receivers and therefore cannot fly LPV or GLS approaches. I think the A350 was the first one with both as standard. Also, many areas in the world are not yet covered by an SBAS; and GBAS is still very rare today.

@Bianfable OK, I'm curious and put that in a question.

@Bianfable that's a good point.

@Pilothead don't get me wrong, I appreciate your answer, I have no problem that you keep your answer, it is good data, is just not what I asked exactly, I wanted to compared ILS to GPS by itself only intentionally.

@Bianfable Many countries have ADS-B out mandates for commercial airliners which require SBAS in order to work properly, and retrofits have been created. The US put a mandate in place a decade ago but allowed many deferrals, the last of which expire this year. Many other countries/areas have as well including Europe, Australia, India, Malaysia, Singapore, Taiwan, Hong Kong and others on the way including China and Canada. The number of commercial aircraft now or in the near future in these places without SBAS is effectively zero. I think it's safe to say unaugmented GPS is obsolete.


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Should approach over water have higher glide path than over ground?
On this video the pilot says he is above the glide path intentionally because the approach to the runway is over water so that he has more altitude in case of engine failure.
However, we know that statistically although a water landing is more dangerous, a forced landing on the ground is not very safe either.  There are houses, buildings, trees, power lines, cars, fire explosion, or you could simply flip over.
My point is, shouldn't one always fly above the glide path(specially on single engine), whether you are flying over water or over ground?

what I mean is that in case of engine failure over ground you also have a lot of risks, so you should fly above glide path the same way this pilot is flying above glide path because he over water.

It is not necessary to fly an approach higher over water than for flying an approach into a landlocked airport day VFR. The guy in the video screwed up and was very, very high on that approach. Night landings at coastal airports near harbors, bays, etc. would make a steeper approach, more prudent for a pilot, simply to avoid the risk of unlighted sailboat masts.  There have been cases of airplanes striking said masts while on final approach.  Peter O’Knight airport (KTPF) on Tampa Bay has had several such accidents Where aircraft struck sailboats in the harbor while flying shallow approaches into the airport.  But there is no rule on the books requiring steeper approaches during a final approach path over water.

So you are saying if you are established on final in the glide path (two red two white) on a cessna 172, normal weather, you can make to the runway with no problem with the engine out?

It’s a fair point, though it is applicable to terrestrial landing at a landlocked airport as well.  I do not prefer to execute normal landings in such a manner.


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Why don't flight attendants wear pants instead of skirts?

After watching this video and noticing these Super Jet Airline flight attendants are wearing pants, I asked myself why not all airlines have their flight attendants wear pants instead of dress/skirts so that they have more easy movement in case of emergency. I also noticed instead of dress/high heels shoes they are wearing a sport shoes which would also make it more versatile in case of emergency. After all, the safety of everyone onboard is their number one role, not serving food, drinks or being pretty(not that you can't do all of those at the same time anyway).
I mean, I can see that airline management probably wants to have a more formal dress code image of their airline, but probably that's something that has stuck trough the decades from the airlines early beginnings where every living soul was wearing dress/suit and air traveling was more for the business class.
If we look at todays airlines, the rise of low budget traveling, and the fact that if you look at most passengers whether it is a low budget airline or not, business class or not, I would say that more than 90% of passengers travel with casual dress code, in other words, in general, the flight attendants dress code doesn't match the ambient(their passengers) anyway.

My point is, does a flight attendant wearing pants is faster/versatile(hence safer) than one wearing a dress and if so why don't we see more often pants attendants, or at least some safety guidance promoting it (if there isn't one already I'm missing)?

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Some do. Despite my attempts, I've never successfully dared a cis male FA to wear a skirt...
As your picture very well illustrates, FA uniforms are a marketing billboard for the airline. Their employer wants them to look a certain way. And most of these employers want skirts and heels.
There is no practical reason for wearing skirts, heels, and nylons aboard an aircraft. The most practical option, both for normal flight and for emergency situations, would be pants and a hoodie. Fabrics are probably a bigger problem than skirts: practical materials would be a merino wool underlayer and a loose rougher wool outer layer with pants, with mid-calf or taller leather footwear. Note that Nomex isn't necessary - it's comparable to wool in fire resistance, but loses on heat regulation and odor resistance.
Early FA attire was almost practical, with flat shoes and natural fabrics, but still with skirts, since it was unusual for high-class women to wear pants in the early 20th century. Stockings these days were also made out of natural fabrics and didn't represent a danger.
Since then, high heels became more popular, and fabrics got worse. Many airlines want bright colors in their branding, which are much easier to achieve and maintain with synthetics than with wool or silk. Synthetics are easy to clean and hard to damage through abuse or improper washing. No shrinkage, no bleeding, no stains, and cheap to replace when hiring/firing.
Most current FA attire is impractical and impairs performance both in normal work, if heels or tight skirts are worn, and in case of emergency. That said, lungs remain the most vulnerable part of the human body. Synthetic fabrics will destroy the skin underneath if exposed to fire, but lung failure prevents the grimy consequences of plastic burns by killing one first. So there isn't a lot of fatalities due to burns, compared to smoke inhalation.
Are more practical or protective uniforms necessary? Depends. While air is the safest form of travel per mile, for professionals averaging thousands of miles per day, accidents do become a non-negligible factor in mortality. With an accident rate of ~1 per million departures, it takes 74 flights per year for an air accident to be as likely as a major driving accident in Europe. That's a lot for a passenger, but on the low end for cabin crew. Flight attendants aren't entertainment; it's a physical job, with a key role in the safety of the passengers aboard.
So it would probably be worthwhile to modernize FA uniforms. The heels, for one, impair performance to the point that they must be removed in an emergency, not to mention long-term health concerns. Gender norms have long since changed towards more unisex clothing. But there isn't any political or regulatory push for such a change at the moment.

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Because hostess needs to be attractive to a specific demography: old rich farts (applies to any gender). Same demography requires stewards to be dressed like penguins.
The Indonesian no-fril airline has no such target customers, and therefore they made a sensible choice for their hostesses.

Well no, they made an incredibly cynical marketing choice. Exactly as companies make, say "green-washing" cynical marketing-based choices to "signal" a certain totally-nonexistent ethic.

Three reasons:

Sex sells.  Ogling female flight attendants wearing heels and skirts that reveal their legs has long been a past-time of a certain audience.  And many airlines executives are undoubtedly members of that particular audience.

Fear of change. Change requires courage.  Historically, airline executives have not demonstrated much courage.  Quite the opposite, in fact.  Historically, they tend to dig their heels in (proverbially) and get defensive when confronted with many industry issues.

Passenger safety is not the biggest priority.  Although a change in flight attendant wardrobe would likely improve passenger safety a little bit (can you respond to an emergency more efficiently in heels or flats?), most airlines prefer profits over safety.  It will likely require forward-thinking people, or an incident in which people die that could have been easily prevented, to change the current stagnant culture.

have their flight attendants wear pants instead of dress/skirts so that they have more easy movement in case of emergency.

The Scottish and Greek traditional dress for men includes skirts because it is easier for them to move (especially when it comes to climbing).
This is just an extra (historical, mostly) data point in the "what is better in case of emergencies" discussion.

The Scottish men indeed  have dress skirt(called kilt) but those are very different than the ones used by flight attendants(they are not tight, as you said, they allow easier movement). Another point is, last time I checked, Scottish men that wear kilt don't wear high heels.

@Gabe this was a semi-serious answer because it does not address the elephant in the room which is to make sure the female crew is sexy.  I am not a historian but heard about the Scots' and Greeks' traditional attire as a true solution for the hilly environments they were dealing with. And that the Greek skirt is called a fustanella :)

yeah I see your point and that's something of what I mentioned in the initial post about the airline management. Having said that, I honestly think a woman could wear pants and still be "attractive" if done correctly, if that's what management wants. But that's just my opinion.

My Scot and Greek friends have been atypically quiet since I sent them your answer.  All these years, they have been insisting it was their ancestor's abnormally large genitalia that demanded they prance around the hills wearing cute little dresses...


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A commercial jet with rear tail-mounted engines and 3+3 seat pattern?
I have just spotted a cameo in one of the old CSI:NY episodes. External shot showed a commercial jet, rather big aircraft, with rear tail-mounted engines. While internal shots shown 3+3 seat rows configuration and around 150-180 passengers.
Is this combination ever possible or is this an in-movie mistake?
I was always told that rear tail-mounted engines are capable of powering smaller aircraft only (like MD-8x and Canadairs) which are always configured in 2+2 seat rows set and are capable of delivering around 100 passengers at most.
What am I missing (note that I am not talking about trijets)?

The interior shot was probably of a different aircraft. DC-9 and MD-80 aircraft are 5 abreast with up to 150 passengers.

The Vickers VC10 was not a "smaller" aircraft.

The Internet Movie Plane Database lists only 2 commercial aircraft for CSI:NY.  A 727 and a 767.

Only thing I could find with tail-mounted engines and 3+3 seating is the Tu-334, but only 2 were ever built and ironically it is smaller (seats about 100).

@fooot: The 727 (one of which, as Gerry says, is explicitly mentioned by IMPDB as having shown up in CSI-NY) had three engines, all tail-mounted, and six-abreast seating, and the ability to carry (for the 727-200) up to 189 passengers.

@Sean but the last line of the question says "note that I am not talking about trijets". Other than that I agree the 727 makes the most sense here.

@fooot: The 727's middle engine is buried in the tailcone and easily overlooked; it's likely that what he thinks was a twinjet was actually a 727 that he didn't notice the middle engine of.

@trejder Your mistake is assuming that the editors of a TV show would care about that sort of technical consistency on something like a secondary scene with filler like that.  The interior shot and exterior shot will have absolutely no connection - the exterior shot being a stock clip grabbed from somewhere by someone with no clue or care.

John K nailed it.  TV and movies are well known for mixing things up like this.

If it was a season 5 episode, it was most likely N7264U, a 727-200:1

Season Five
Boeing 727-222
Ep. 5.25
Registration N7264U, c/n 21411/1346 built in 1978 for United. Based at Ontario International Airport (ONT/KONT) in California, where this was most likely filmed.

The 727's three Pratt & Whitney JT8D engines are all mounted on/in its tail, and the plane was large enough to have six-abreast (3+3) seating, as can be seen in this 727-200 seatmap found via a quick google (this particular seatmap is for an EAL 727, but the 3+3 seating was common to all passenger 727s of which I am aware):

(Image by Eastern Airlines, via Darren at Frequently Flying.)
And the 727-200's maximum passenger capacity was 189, according to Wikipedia, although it more usually carried 134 (in a two-class configuration) to 155 (all-economy arrangement), so it fits the 150-180-passenger requirement (and even exceeds it slightly at the upper end).
And yes, I know it's a trijet, but:

the #2 engine is buried within the 727's tailcone and mostly concealed from view, unlike the large, obvious pods holding the #1 and #3 engines (as opposed to, say, a DC-10, whose centerline engine and associated intake duct form a huge conspicuous cylinder above the fuselage at the base of the vertical stabiliser);
as for the centerline engine's tailpipe (which is visible, sticking out the tip of the tail, if you look carefully), the JT8D is a slim low-bypass turbofan, making for a much narrower and less obtrusive tailpipe compared to the chunky high-bypass engines that occupy the tailcone of (say) an L-1011 or Yak-42;

...and, putting both of those together, and considering that it was a (presumably brief) cameo giving no reason, and little opportunity, for you to pay close attention to the tip of the aircraft's tailcone, it's perfectly possible that what you saw was, in fact, a (trijet) 727, and that you simply didn't notice the easy-to-miss-at-a-brief-glance middle engine.

1: Thanx to Gerry for providing the link.


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Can the dome gunner on a B-17 Flying Fortress shoot the plane's tail off?
I was at the California Capital Airshow on Mather AFB over the weekend, and came across a beautifully maintained B-17 Flying Fortress. The crew were kind enough to allow people to go inside and take a look - what a magnificent machine!

While inside, I noticed the dome gunner seems to be in a position to accidentally shoot the tail off the plane!

Are there any confirmed instances of this happening? Was there some way to prevent it - either be it training or some mechanism that stopped the guns while in this position? I can imagine, in the heat of battle, sweeping an enemy fighter and nicking your own tail in the process!

This page contains an account of a waist gunner shooting the tail, but I'm unable to do more research into it at the moment.

@RonBeyer Oh wow, good find! 'The entry in the pilot’s diary, dated Feb 18th, 1943, says, “New waist gunner shot hell out of tail today. Ship out for a week.”'

If your not careful thats what happens at 11 0'clock

IIRC shooting your own aircraft was a serious risk with pintle mounted machine guns in early WW1 aircraft.

My grandfather was an air Sargent and gunner on B-17.  He told me a story where he had accidentally shot down his plane because the ball turret gun cutoffs were wired shut. These cut-offs were to prevent hitting the aircraft was the turret guns rotated but German fighters quickly learned to approach bombers these angles. This led to crews wiring shut the cut-offs.  Fortunately for my grandfather, being the air Sargent, he was responsible for the combat log for the mission and also enemy fighters were encountered so he could report the plane was hit by enemy fire.

@DanNeely That's correct. It took the invention of Fokker's interrupter, which automatically repressed fire when a blade was in front of the gun, to prevent this.

That second comment above deserves to be incorporated into an answer; great info, ditto the second comment down from top.

There's a profile cam in the turret track ring that operates a mechanism that interrupts the guns when the barrels are pointed at parts of the aircraft.  Waist gunners were the only ones who had to worry about hitting their own plane.
The bigger problem was gunners hitting adjacent aircraft.  The "box" formation design attempted to provide as much of an open field as possible to each gunner.

Not just gunners hitting friendly air craft - the bombs themselves have hit planes flying below them as well.  Recall watching old WW2 documentaries w/ various gun cam footage and seeing it happen...

Didn't the top turret gunner also have the potential (if there were no interrupter mechanism) to shoot part of the plane, as shown in the question?

@WayneConrad as far as I know yes the B-17 specifically did have such a feature in the top gunner position. I think that's what John K meant, but there are bombers with such positions that did not have any such mechanism, ahem some German bombers.

It's this statement that I'm confused about: "Waist gunners were the only ones who had to worry about hitting their own plane."  I wonder if John meant to write "were not the only ones."

The only gunners that had the ability to shoot directly into their own airplane's structure were the waist gunners.

@JohnK Would this then be accurate?  "Because the top turret gun had an interrupter mechanism, it could not shoot the airplane even though it could aim at the tail.  However, the waist guns, which had no interrupter mechanism, could be aimed at the airplane.  Therefore, the waist gunners were the only ones who had to worry about hitting their own plane."

How large was the "shadow" created by the cam mechanism?  Did it create a "safe area" for ememy aircraft, or was the area created by the cam not really any bigger than that created by the tail itself?

@supercat - that's a great new question - why don't you ask it!

@FreeMan: I would expect that anyone with information about the size of the "shadow" would also be reading this question, and many people interested in this question would be curious about how accurately the shadow produced by the mechanism matches the shadow produced by the tail.

@supercat you'd be amazed at the knowledge and mad Google skillz of this community! Also, just because it's only interesting to you doesn't mean it's not a good question. It would also be interesting to me, and a lot of others would read it and think, "Huh... that's a good question! Wish I'd thought of it!"

@FreeMan: It is sufficiently related to this question that if anyone would know the answer, I would think it should be included in an answer to this question.

@supercat A good way to deal with a related question like that is to ask it as a separate question, but in the question body, link to the related question/answer.

@supercat This comment by Gary Kindel indicates that the shadow was substantial.

If you fish around on the web you can prolly find a Sperry or Bendix operating manual.